The present invention relates to an image processing apparatus for quantizing multi-level image data to binary image data.
Conventionally, an image processing method as disclosed in Japanese Patent Application Laid-Open No. 2-210959 has been proposed as a pseudo-half-tone processing method for a digital copying machine, a facsimile machine and the like. In this method, an object pixel is binarized to a black or white pixel in accordance with binary data of neighboring pixels of the object pixel which have been already binarized, and errors produced during the binarization process are added to the pixel data of other neighboring pixels which are not binarized yet.
This method has an advantage that the processing volume can be saved for an calculating operation of average density data as a threshold value for binarization, because only the binary data of the binarized neighboring pixels are required for the process. Further, as errors between input multi-level data and the average density data generated in the binarization of the input multi-level data are corrected, image data with excellent tonality can be obtained.
However, the above conventional method where the object pixel is binarized with the condition of neighboring pixels has the following drawbacks:
I. Overlaps of dots (some dots become too close to each other), etc. in highlight portions of the resulting reproduced image data deteriorate image quality; PA1 II. When screened-dot image data are binarized, moire patterns are produced in the dot patterned area; and PA1 III. A memory for storing error data is required because errors generated in binarization are added to neighboring pixels which have not yet been binarized. PA1 input means for inputting multi-level image data corresponding to an object pixel; PA1 random number generating means for generating a random number; PA1 first binarization means for binarizing a first portion of the multi-level image data input by said input means in accordance with the random number generated by said random number generating means; PA1 modifying means for modifying second portion of the multi-level image data input by said input means in accordance with the binarized data obtained by said first binarization means; PA1 second binarization means for binarizing said second portion modified by said modifying means in accordance with the data of the neighboring pixels of said object pixel binarized by said first binarization means; and PA1 output means for outputting the binarized data obtained by said first and second binarization means as binary data of the object pixel. PA1 first binarization means for binarizing m (m&lt;n) lower-order bit data of the multi-level image data of the object pixel; and PA1 second binarization means for binarizing the multi-level image data quantized by the binarization of said m lower-order bit data by said first binarization means in a manner different from that of said first binarization means. PA1 means for generating a uniform random number; and PA1 means for binarizing said m lower-order bit data in accordance with a generated random number. PA1 means for calculating a weighed mean value of surrounding pixels of said object pixel in accordance with the binarized data of neighboring pixels of the object pixel, which have been already binarized prior to binarization of said object pixel; and PA1 means for binarizing the quantized multi-level image data in accordance with said weighed mean value.